Check if Cascading's co-group iterator exposure lets us reimpl MultiGroupBy

[sritchie]

From @cwensel:

just added a commit that grants access to the co group iterators from a Buffer https://github.com/ConcurrentCore/cascading/commit/2e3a1a7c93a012ef15e690a6728e191d1194b127

this test shows how it works https://github.com/ConcurrentCore/cascading/commit/2e3a1a7c93a012ef15e690a6728e191d1194b127#L31R182